Hydrosilylation chemistry, involving the reaction between a silylhydride and an unsaturated organic group, is the basis for synthetic routes to produce commercial silicone products such as silicone surfactants, silicone fluids and silanes as well as many addition cured products including sealants, adhesives, and silicone-based coatings. Conventionally, hydrosilylation reactions have been typically catalyzed by precious metal catalysts, such as platinum or rhodium metal complexes.
Various precious metal complex catalysts are known in the art. For example, U.S. Pat. No. 3,775,452 discloses a platinum complex containing unsaturated siloxanes as ligands. This type of catalyst is known as Karstedt's catalyst. Other exemplary platinum-based hydrosilylation catalysts that have been described in the literature include Ashby's catalyst as disclosed in U.S. Pat. No. 3,159,601, Lamoreaux's catalyst as disclosed in U.S. Pat. No. 3,220,972, and Speier's catalyst as disclosed in Speier, J. L, Webster J. A. and Barnes G. H., J. Am. Chem. Soc. 79, 974 (1957).
Although these precious metal compounds and complexes are widely accepted as catalysts for hydrosilylation reactions, they have several distinct disadvantages. One disadvantage is that the precious metal complex catalysts are inefficient in catalyzing certain reactions. For example, in the case of hydrosilylations of allyl polyethers with silicone hydrides using precious metal complex catalysts, use of a large excess of allyl polyether, relative to the amount of silicone hydride, is needed to compensate for the lack of efficiency of the catalyst in order to ensure complete conversion of the silicone hydride to a useful product. When the hydrosilylation reaction is completed, this excess allyl polyether must either be: (A) removed by an additional step, which is not cost-effective, or (B) left in the product which can result in reduced performance of this product in end-use applications. Additionally, allyl polyether hydrosilylation with conventional precious metal catalysts typically results in a significant amount of undesired side products such as olefin isomers, which in turn can lead to the formation of undesirably odoriferous byproduct compounds.
Further, due to the high price of precious metals, catalysts derived from these metals can constitute a significant proportion of the cost of silicone formulations. Over the last two decades, global demand for precious metals, including platinum, has sharply increased, driving prices several hundred folds higher, thereby precipitating the need for effective, low cost replacement catalysts.
As an alternative to precious metals, certain iron complexes have been disclosed as suitable for use as ketone and/or aldehyde hydrosilylation catalysts. Illustratively, technical journal articles have disclosed that iron salts treated with phosphine or nitrogen compounds catalyze the hydrosilylation reaction of activated double bonds such as aldehydes and ketones at long reaction times. (Beller et al. Organic Letters, 2007, 26, 5429-5432; Beller et al. Angew. Chem. Int. Ed., 2008, 47, 2497-2501; Nishiyama et al. Tetrahedron Letters, 2008, 49, 110-113) Nishiyama (Chem. Commun. 2007, 760-762) reported the poor activity of iron acetate and 2,6-(2,4,6-Me3-C6H2N═CMe)2C5H3N (MesPDI) in the hydrosilylation of methyl(4-phenyl)phenylketone (7% after 20 h at 65° C.).
Certain iron complexes have also been disclosed as suitable for use as alkene hydrosilylation catalysts. For example, Fe(CO)5 has been shown to catalyze hydrosilylation reactions at high temperatures: Nesmeyanov, A. N. et al., Tetrahedron 1962, 17, 61; Corey, J. Y et al., J. Chem. Rev. 1999, 99, 175; C. Randolph, M. S. Wrighton, J. Am. Chem. Soc. 1986, 108, 3366). However, undesirable by-products such as unsaturated silyl-olefins, resulting from dehydrogenative silylation, were formed as well.
A five-coordinate Fe(II) complex containing a pyridine di-imine (PDI) ligand with isopropyl substituents at the ortho positions of the aniline rings has been used to hydrosilylate an unsaturated hydrocarbon (1-hexene) with primary and secondary silanes such as PhSiH3 or Ph2SiH2 (Bart et al., J. Am. Chem. Soc., 2004, 126, 13794; Archer, A. M. et al. Organometallics 2006, 25, 4269). However, one limitation of these catalysts is that they are only effective with the aforementioned primary and secondary phenyl-substituted silanes, and not with, for example, tertiary or alkyl-substituted silanes such as Et3SiH, or with alkoxy substituted silanes such as (EtO)3SiH.
Recently, new and inexpensive Fe, Ni, Co and Mn complexes containing a terdentate nitrogen ligand have been found to selectively catalyze hydrosilylation reactions, as described in co-pending U.S. Pat. Nos. 8,236,915 and 8,415,443. Chirik has reported the formation of the catalytically inactive Fe(PDI)2 species. The undesired Fe(PDI)2 is formed by treatment of PDIFeBr2 with the reductant Na(Hg). The yield of the Fe(PDI)2 species increases when the reduction is performed in the presence of excess free PDI. (Chirik et al. Inorg. Chem. 2010, 49, 2782-2792. Chirik et al. Inorg. Chem. 2009, 48, 4190-4200).
One restriction of these new non-precious metal based catalysts, however, is that they are normally extremely sensitive to air and moisture, and thus are unlikely to perform well or consistently if exposed to air or moisture prior to their use. For this reason, these catalysts are typically prepared and stored under hermetically inert conditions such as in a dry box. Since it is impractical to install and use such highly inert-atmosphere equipment widely on an industrial scale, the use of these catalysts in a commercial setting may be economically prohibitive. Accordingly, there is a need in the industry for non-precious metal-based catalysts that do not require manufacturing and storing under inert conditions.
Methods are known in the art to activate catalyst precursors in-situ. The most well-known example is the activation of Ziegler-Natta catalyst by methylaluminoxane (MAO) for the production of polypropylene from propene (Y. V. Kissin Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier, 2008, Chapter 4).
Additional examples of catalyst activation are also known. U.S. Pat. No. 5,955,555 discloses the activation of certain iron or cobalt PDI dianion complexes by polymethylaluminoxane (PMAO) for olefin polymerization. U.S. Pat. No. 4,729,821 discloses the in-situ activation of Ni-catalysts by applied electrical potentials for the hydrogenolysis of ethane and ethylene. Martinez et al. demonstrated the in-situ activation of a [RuCl2(p-cym)]2 complex by phosphine ligands in a C—C bond formation reaction via C—H bond activation of aryl-compounds (J. Am. Chem. Soc, 2009, 131, 7887). Yi et al. described the in-situ formation of cationic ruthenium hydride complexes which catalyze the regioselective intermolecular coupling reaction of arylketones and alkenes involving C—H bond activation (Organometallics, 2009, 28, 426). More recently, Thomas et al. have described the activation of base metal complexes with ethyl magnesium bromide (Adv. Synth. Catal. 2014, 356(2-3), 584-590).
The in-situ activation of non-precious metal-based catalysts for alkene hydrosilylation reactions has been described. (See, U.S. Patent Application Publication 2012/013106A1). However, this activation employs NaBEt3H, which is sensitive to air and incompatible with alkoxysilanes. Alkoxysilanes are known to undergo dangerous disproportionation reactions with strong hydride donors such as alkali borohydrides or alkali hydrides. (Woo, H.; Song, S.; Cho, E.; Jung, I.; Bull. Korean Chem. Soc. 1996, 17, 123-125. Itoh, M.; Ihoue, K.; Ishikawa, J.; Iwata, K. J. Organomet. Chem., 2001, 629 1-6.)
There is a continuing need in the hydrosilylation industry for methods of activating non-precious metal catalysts using milder reducing reagents that are also compatible with alkoxysilanes. The present invention provides one solution toward that need.